Capacitor assisted starting with a soft starter
Posted 22 May 2005 - 05:54 PM
I have just seen a system designed by a consulting engineer, possibly with assistance from a soft starter manufacturer. The application is a large pump motor that must be accelerated with a "guaranteed" 200% FLA maximum starting current. While theoretically possible, I already told them it was not probable. However, they put it out to bid with documents calling for a competitor's product. When I looked at the one-line diagram, I saw the engineer has 2 sets of capacitors downstream from the SCR stack of the soft starter with isolation contactors to bring them in separately; one labeled "PFC caps" (which I understand), but the other labeled "Starting Caps" which I have never seen before.
My first reaction was to jump all over it as having the caps downstream from the SCRs at all, but them I noticed a small inductor symbol in the power line between the SCRs and the caps, indication to me that the engineer (or someone) knew that he would need inductance in order to keep the caps from popping due to the steep wavefront of the SCR switching.
My question however, is what possible benefit could this add to the system? I know that capacitor assisted starting is a rudimentary method of attempting to keep starting currents lower in DOL starters for large motors by supplying starting VARs, but what good would that do with a soft starter output? And would the voltage drop across the inductor cause a loss in torque that would negate any perceived benefit of having the starting caps?
Has anyone ever heard of this before?
If so, why isn't it used all the time?
Posted 22 May 2005 - 07:17 PM
This is something I have considered previously, and I can not see it being a practical solution.
One of the problems is that in order to work, you need to have a continuously variable capacitance to neutralize the VARs of the motor being started. Unlike the running situation where the VARs are essentially load independent, during start the VARs are speed dependent.
Another issue would be the charging current into the capacitors each time the SCRs switch ON. An inductor would reduce the initial inrush at switch on, but the peak current would stilll be high as the capacitor would still require the full charge.
You would also need to be very careful not to go near resonance with the capacitors and the motor.
I have not seen any information suggesting that this is a workable solution, and frankly, not having actually tried it, I am somewhat skeptical about it as a solution.
There may be a solution, but I am not convinced that it would operate as described.
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Posted 22 May 2005 - 07:39 PM
FWIW, here is the one-line drawing in question.
Notice that the engineer just put ? next to the capacitor sizes!
"Make this work, fill in the details..."
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